Eaters of the Dead, by Michael CrichtonRead about another entry on the list.
Crichton always surprised; in this 1975 novel (which served as the inspiration for the film The 13th Warrior) a civilized, educated Arab courtier falls in with a group of Viking warriors. He finds them gross and brutal—and slowly realizes they have taken him in order to assist in a battle against the beasts that emerge from the darkness to consume them (think Grendel). While Crichton offers a pseudo-scientific explanation for these events, the old myths loom large in what is a pretty scary story.
--Marshal Zeringue